In recent years, hybrid vehicles (HV), plug-in hybrid vehicles (PHV), and electric vehicles (EV) are being spread. Secondary batteries as a key device are installed in these vehicles. As secondary batteries for the vehicle, the nickel hydride batteries and the lithium ion batteries are spread. In the future, it is expected that spread of the lithium ion batteries having high energy density are accelerated.
Since the operable voltage range and the prohibited voltage range in the lithium ion batteries are close, the stricter voltage management is necessary in the lithium ion batteries than other types of batteries. When an assembled battery in which a plurality of the lithium ion battery cells are connected in series is used, a voltage detection circuit is provided for detecting each of the battery cells. In a case where the number of the battery cells is large, a plurality of cascade-connected voltage detection circuits are used. The voltage detected in each of the battery cells is used for controlling of charge or discharge, equalization in the cell voltages, or the like.
Usually, it is designed that the power source for the plurality of the voltage detection circuits is supplied from the assembled battery. When it is designed that the power source is supplied from other than the assembled battery, a volume of the circuit is increased. In the following in this specification, the power source for the plurality of the voltage detection circuits is supplied from the assembled battery.
Detecting the disconnection of the voltage detection lines in the assembled battery is a required item of failure detection. When a current does not flow through the voltage detection line, it is difficult that the disconnection of such a line is detected. When the plurality of the voltage detection circuits are cascade-connected, in adjacent two of the voltage detection circuits, a ground terminal of the higher-potential side voltage detection circuit and a power source terminal of the lower-potential side detection circuit are connected, and its connected node and a node connected between the battery cells corresponding to the connected node are connected through the voltage detecting line
In this case, a current flows from the higher-potential side voltage detection circuit to the lower-potential voltage side detection circuit. In some cases of this current value, it happens that a very little current flows through the voltage detecting line of the above-mentioned node between the battery cells. In a case where the disconnection of the voltage detection line happens in this state, it is difficult to detect the disconnection of the voltage detection line, because detected voltage values hardly change. In contrast, the impedance ratio between the adjacent voltage detection circuits is adjusted, and then a method that electric potential difference between both end of the voltage detection line is increased is proposed (for example, refer to Patent Literature 1).